The big screen adaptation of The Mortal Instruments: City of Bones hasn't even been released yet, but the studio is betting on its success. Constantin Films has already started to develop a spin-off film, based on the spin-off book series called Infernal Devices, which was written by Mortal Instruments author Cassandra Clare.
Mortal Instruments centers on a young girl who discovers she is part of a hidden race of angels known as Shadow hunters locked in a battle with warlocks and vampires called Downworlders.
Infernal Devices is a prequel that is set in Victorian England and follows a new teen girl plunged into a journey of adventure and self-discovery. These stories feature the ancestors of several Mortal Instruments characters, and they even include some character overlap. The book titles are Clockwork Angel, Clockwork Prince, and Clockwork Princess. Stephanie Sanditz (Kate & Leopold) has been hired to write the screenplay.
I'm not really familiar with these books, but then again I'm not part of the target audience for them. A panel was held for City of Bones at WonderCon a couple of weeks ago, and there was a ton of fans that showed up for it. Apparently the series has a huge fanbase so these movies might end up doing well.